1. Field of Invention
The present invention relates to wireless communications, and more particularly, to managing the radio link timer in a wireless communications network to control when a call between a mobile station and a base station should be released due to radio link failure.
2. Description of Related Art
When a call is in progress in a wireless network, a radio link timer (RLT) in the mobile station (MS) and a RLT at the base station system (BSS) control when calls between the MS and BS are to be released because of poor radio quality and/or radio link failure. When the RLT at either the MS or BS times out, the call is released. Both the MS and the BS have RLT management logic that monitors the radio link. In one common implementation, the RLT management logic monitors the number of control channel (CCH) frames that have been successfully decoded during a time interval and makes a determination as to whether the call should be dropped based on the number of CCH frames that have been successfully decoded. In global system for mobile communications (GSM) wireless networks, the CCH frames that govern RLT management are called Slow Associated Dedicated Control Channel (SACCH) frames. The SACCH channel carries call signaling information such as, for example, connection management information, mobility management information and radio resource information.
Both speech frames and SACCH frames are transmitted between the MS and the BS during a call. Decoding circuitry of the MS decodes the frames and passes the decoded information on to call processing function of the MS. In GSM wireless networks, the RLT management logic of the MS includes a counter that is decremented by 1 when the MS decoding circuitry is unable to successfully decode a SACCH frame and increments the counter by 2 (up to a maximum value equal to the initial setting of the counter) when the MS decoding circuitry is able to successfully decode a SACCH frame. The initial value of the counter can be configured by the wireless network parameters. When the counter is decremented to 0, the MS releases the call. The counter cannot be incremented to a value that is greater than its initial value. This particular RLT incrementing and decrementing algorithm results in a call being released when more than 66.66% of the SACCH frames are unable to be successfully decoded.
At the BSS, which typically includes the base transceiver station (BTS) and the base station controller (BSC), the BSC typically controls RLT management. The RLT management logic at the BSC includes a counter that starts at some initial value, such as 24, for example. The RLT of the BSC decrements the counter by 1 when the decoding circuitry of the BS is unable to successfully decode a SACCH frame. The RLT increments the counter by 2 (up to a maximum value equal to the initial setting of the counter) when the BSC decoding circuitry is able to successfully decode a SACCH frame. When the counter is decremented to 0, the BS releases the call. The counter cannot be incremented to a value greater than its initial setting, which is generally specified by configurable wireless network parameters. Like the MS, the incrementing and decrementing algorithm performed by the BSC results in a call being released when the BSC is unable to successfully decode more than 66.66% of the SACCH frames.
In GSM systems, speech frames are encoded by a Half Rate, Full Rate, Enhanced Full Rate (EFR) or Adaptive Multi Rate (AMR) vocoder. The EFR vocoder is governed by a particular standard that requires sampling speech at a bit rate of 12.2 kilobits per second (kbps). Once the speech has been sampled, error bits are added for error coding. Encoded speech bits and encoded SACCH bits are combined into the same frame, and the encoding of the SACCH frame bits is comparable to the encoding of the speech bits. Consequently, if more than 66.6% of the SACCH frames cannot be successfully decoded, it is quite likely that the voice quality has degraded to the point that the call should be released. However, for EFR vocoders, it is also possible that while a sufficient number of SACCH frames are able to be successfully decoded to prevent the RLT from timing out, voice quality is unintelligible. Therefore, with EFR vocoders, calls are not always released when they should be released.
Like the EFR vocoder, the more recently implemented AMR vocoder is governed by a particular standard that calls for sampling speech at anywhere from 12.2 kbps down to 4.75 kbps, depending on whether less error correction bits or more error correction bits, respectively, are added to the frames. When signal quality is very good, the AMR vocoder samples at the higher sampling rate of 12.2 kbps, but as signal quality degrades, more error correction bits are added to the frames and less speech bits are included. When signal quality is relatively weak, the AMR vocoder could sample speech at the lower rate of 4.75 kbps, and thus add a larger number of error correction bits to the frames. Encoded SACCH bits are added to the frames on top of the encoded speech bits.
With AMR, SACCH encoding does not change as speech sampling and encoding adapt to the measured signal quality. Nevertheless, the same RLT algorithm that is used for EFR is also used for AMR. When the AMR vocoder is sampling speech at the lower rate, such as, for example, 4.75 kbps, it is possible that more than 66.6% of SACCH frames are unable to be successfully decoded, but that speech can be successfully decoded at the lower rate speech codecs, which means that voice quality is satisfactory. Nevertheless, when more than 66.6% of the SACCH frames are unable to be successfully decoded, the RLT will time out and the call will be released even though voice quality is intelligible.
Accordingly, a need exists for a method and apparatus for performing RLT management that prevent calls from being released by a radio link timer management module when voice or data quality is intelligible and that ensure that calls are released when voice or data quality is unintelligible.